1. Field of the Invention
The invention pertains generally to a method for fabricating articles which include high silica glass bodies, e.g., high silica glass optical fibers, as well as the articles produced by the method.
2. Art Background
Articles which include high silica glass bodies, i.e., glass bodies containing at least 70 percent by weight of silica, are currently employed in a wide variety of commercial settings. For example, optical fibers, drawn from high silica glass optical fiber preforms, are currently being used in optical communication systems. Such fibers typically include a high silica glass core encircled by a high silica glass cladding, with the former having a higher refractive index than the latter to achieve guiding of electromagnetic radiation. This difference in refractive index is achieved, for example, by incorporating an up dopant (a dopant which increases refractive index) into the core, or incorporating a down dopant (a dopant which decreases refractive index) into the cladding, or through the incorporation of both up and down dopants. Other articles which include high silica glass bodies, such as high silica glass lenses and prisms, are used in a wide variety of optical systems, while articles such as high silica glass refractory tubing, muffles and holders are employed in the heat treatment and processing of semiconductors.
A number of techniques have been developed for fabricating high silica glass bodies. In perhaps the most widely used of these techniques, naturally occurring quartz crystals are initially hand sorted, and then heated to the alpha-beta quartz transition temperature (approximately 573 degrees Centigrade (C) to fracture the sorted crystals. The fractured quartz is again hand sorted and then crushed, typically in a ball milling machine. After being cleaned, e.g., acid washed, the crushed quartz is then introduced into an oxy-hydrogen flame to fuse the quartz powder into a high silica glass body.
While the above-described technique is useful, it does have a number of disadvantages. For example, this technique is relatively expensive because of the need to select (i.e., sort) the raw material (the naturally occurring quartz) by hand. In addition, the raw material typically contains impurities, such as iron ions and other transition metal ions, as well as hydroxyl groups, which absorb electromagnetic radiation of wavelengths equal, or close, to those employed in commercial optical fiber communication systems, e.g., 1.3 micrometers (.mu.m), and thus produce relatively high optical loss. Moreover, the raw material often contains other impurities, such as zirconia, which cause scattering and/or produce crystalline phases, e.g., zircon, which degrade the mechanical strength of glass fibers. Additional such scattering impurities are also introduced during the ball milling process, while additional hydroxyl ions are introduced by the oxy-hydrogen flame. Further, this particular glass fabrication technique generally precludes the incorporation of dopants into the resulting glass body. As a consequence, this technique is generally viewed as being undesirable for the fabrication of certain high silica glass bodies, including optical fiber preforms.
Techniques have been developed which avoid at least some of the disadvantages, discussed above, and which thus permit the fabrication of high silica glass bodies such as optical fiber preforms. Two such related techniques are known as the outside vapor deposition (OVD) technique and the vapor-phase axial deposition (VAD) technique. In both techniques, reactive gases, such as SiCl.sub.4 and O.sub.2, are flowed into an oxy-hydrogen flame where they react to form particles of silica, called soot particles, which are thermophoretically deposited onto a glass substrate. If, for example, it is desired to increase the refractive index of the resulting glass body by incorporating up dopants such as GeO.sub.2 or P.sub.2 O.sub.5, then the reactive gases will typically also include GeCl.sub.4 or POCl.sub.3 (which react with the O.sub.2 to form the up dopants). In any event, the resulting, relatively porous soot mass is then heated to the sintering temperature (typically about 1400 to about 1500 degrees C,) to form a relatively dense, high silica glass body.
As discussed, both the OVD and the VAD techniques permit the incorporation of dopants into glass bodies, and are thus useful in the fabrication of, for example, optical fiber preforms. However, the rate of deposition of soot particles in these techniques is relatively low because the deposition rate is limited both by thermophoresis and by the relatively low concentration of silica particles in the gas streams heated by the oxy-hydrogen torch. As a consequence, the resulting glass bodies are relatively expensive.
Another technique, useful in the fabrication of optical fiber preforms, is known as the chemical vapor deposition (CVD) technique. Here, reactive gases, such as those discussed above, are flowed into a silica substrate tube, and allowed to diffuse to the inner surface of the tube where they react to form relatively dense silica glass. Unfortunately, the rate of glass formation is relatively low. Further, attempts to increase the rate of glass formation by increasing the concentrations of the reactive gases have failed because such relatively high concentrations lead to gas phase nucleation of silica particles, which are often swept out of the substrate tube by the gas stream, rather than being deposited onto the inner surface of the substrate tube. Moreover, attempts to increase the rate of glass formation by increasing the flow rate of the reactive gases have been thwarted because at these relatively high flow rates there is insufficient time for the reactive gases to diffuse to the inner surface of the substrate wall (to react and form silica) before being swept out of the substrate tube. Consequently, this technique is also relatively expensive.
Yet another technique useful in the fabrication of high silica glass bodies, such as high silica glass optical fiber preforms, is known as the modified chemical vapor deposition (MCVD) technique. This technique differs from the CVD technique in that silica particles are intentionally nucleated in the gas phase, and thermophoretically deposited onto the inner surface of the substrate tube. This technique is advantageous because it yields high purity glass, and permits the ready incorporation of dopants. However, and although the rate of glass formation is significantly higher than that associated with the CVD technique, and the resulting glass bodies are thus less expensive than those produced via the CVD technique, still higher rates of glass formation, and still less expensive glass bodies, are being sought.
A relatively new glass-forming technique, known as the sol-gel method, offers the possibility of fabricating relatively inexpensive high silica glass bodies. In one variant of this technique, known as the alkoxide gel method, a silicon-containing alkoxide, such as tetraethyl orthosilicate (TEOS), is mixed with a water-containing solution. Because TEOS is normally not miscible with water, mixing is achieved by, for example, dissolving the TEOS in a water-soluble solvent such as ethanol, and then adding the resulting TEOS-ethanol solution to the water-containing solution. This mixing process results in the formation of a sol, which is then poured into a mold to undergo a gelation process. (A sol, for purposes of this disclosure, denotes a combination of liquids, dissolved solids and/or fine particles dispersed in a liquid.) Depending upon a number of variables, the gelation process yields either a silica-containing, porous gel body (with the pores containing liquids such as water and ethanol), or a silica-containing powder which precipitates out of solution. (A gel body, for purposes of this disclosure, is a multiphasic body, i.e., a body which includes at least a liquid and a solid phase, formed from a sol via the interconnection of solid material.) If, for example, the gelation process yields a gel body, then this body is typically dried (to remove the liquids remaining within the pores of the body) and then sintered to form a densified, silica-containing glass body. (Regarding the alkoxide gel method see, e.g., S. Sakka, Treatise on Materials Science and Technology, Vol. 22, Glass, III (Academic Press, New York, 1982).)
Significantly, the starting materials employed in the alkoxide-gel method are typically of relatively high purity, and thus the resulting glass bodies are of equally high purity (the presence of impurities being undesirable because they lead to scattering and/or optical absorption). In addition, index-changing dopants are incorporated into the glass bodies either during the formation of the sol, during the gelation process, or after the gel body has been dried and is still porous. Further, after the drying procedure, water (and thus hydroxyl ions) remaining within the pores of the dried gel bodies are readily removed by contacting the bodies with (gaseous) chlorine. Consequently, the alkoxide-gel method offers many advantages when compared with the other glass-forming techniques. However, large shrinkages occur during drying, and therefore the drying process must generally be carried out at a relatively slow rate to avoid cracking the gel bodies. Moreover, relatively large glass bodies (glass bodies having a mass of a few hundred grams) are not readily achieved.
In a second variant of the sol-gel method, known as the colloidal gel method, commercially available fumed silica, or silica powder formed via the alkoxide-gel method, is mixed with water, and the mixture is cast, gelled, and then dried and sintered. (Regarding the colloidal gel method see, e.g., E. M. Rabinovich et al, Journal of the American Ceramic Society, Vol. 66, p. 683, 1983 and D. W. Johnson, Jr. et al, ibid, p688.) In addition to having many of the advantages of the alkoxide-gel method, the second variant also permits the ready fabrication of relatively large glass bodies, i.e., glass bodies having a mass of a few hundred grams. However, very large glass bodies, i.e., bodies having a mass equal to or greater than about 1 kilogram, are not easily achieved.
High silica glass bodies have also been formed by using a plasma torch to fuse gel-derived silica powders (in this regard see U.S. Pat. No. 3,954,431 to Fleming, Jr. et al). That is, the sol-gel method was used to form a gel body which was dried, and then crushed, to form a silica powder. To eliminate silica particles which were either undesirably large or undesirably small, the silica powder was passed through both a 20 mesh screen as well as a 100 mesh screen. The screened powder was then flowed, via a carrier gas, to a bait placed in the path of the plasma flame, where the powder was melted and fused.
While the above-described plasma torch technique is advantageous, this technique involves the crushing of a dried gel body, which permits relatively little control over the sizes of the resulting powder particles. This lack of control is significant because each plasma torch configuration (and, in fact, each configuration of any type of heat source) permits the melting and fusion of silica particles having only a specific, corresponding size range, i.e., particles outside this specific range are either not incorporated into the glass body being formed or, if incorporated, yield undesirable seeds or bubble defects in the body. As a consequence of this relative lack of particles size control, the above-described plasma torch technique is relatively inefficient in the use of the powder feed stock, i.e., the undesirably large or undesirably small powder particles must necessarily be discarded, and thus much of the powder is wasted.
Thus, those engaged in the development of glass fabrication techniques have sought techniques for forming glass bodies which permit improved sizing control over, and thus relatively efficient use of, the feed-stock, are relatively inexpensive, avoid the incorporation of impurities which cause absorption and scattering, permit the incorporation of index-changing dopants into the glass bodies, and permit the ready fabrication of very large glass bodies, i.e., bodies having masses equal to or greater than about 1 kilogram.